There is an old Hollywood story, probably apocryphal, about Nicholas Ray climbing a giant sand dune on a remote location in the Sahara while directing Richard Burton in Bitter Victory 50 years ago. Reaching the top, he bumped into Henry Hathaway who was making the John Wayne movie Legend of the Lost in the adjoining wadi. With movies currently being shot all over Africa, there's a good chance of something similar happening again. Perhaps Spike Lee, shooting a biopic of Patrice Lumumba in Congo, will bump into Brian De Palma doing a remake of The African Queen co-starring Brad Pitt and Angelina Jolie.

It's no coincidence that all my films of the week this year have been shedding light on the Dark Continent - Alejandro Gonzalez Iñarritu's Babel, shot partly in Morocco, Kevin Macdonald's Last King of Scotland, made largely in Uganda, and now Edward Zwick's Blood Diamond, which uses locations in Sierra Leone, South Africa and Mozambique. The first two are rather complex in the responses they seek to evoke. Blood Diamond is more straightforward. The plot turns on two youngish men, one black, one white, searching for a great hidden treasure.

In another age, it would have been an adventure yarn much in the manner of Rider Haggard's King Solomon's Mines. Here, it's a political thriller with international ramifications (though much simpler than last year's Syriana) and the tale is moulded on an armature of well-researched journalism. The recent African picture it most resembles is The Constant Gardener

The film is set in Sierra Leone in the 1990s and has three well-presented and well-played central characters: Solomon Vandy, a Mende fisherman (played by Djimon Hounsou, the most handsome, beautifully built actor at work today); Danny Archer (Leonardo DiCaprio), a 31-year-old South African; and Maddy Bowen (Jennifer Connelly), a liberal journalist and war chaser, most recently in Bosnia.

Solomon's wife and daughter have found refuge from the civil war in a refugee camp in Guinea, his abducted 10-year-old son has been turned into a murderous boy soldier by the bloodthirsty revolutionary army and Solomon has been made to work as a slave in the rebels' illegal diamond mine. Danny is a self-styled soldier of fortune, who left Rhodesia as a child following the murder of his parents by Mugabe's guerrillas, served with the South African army in Angola, then became a mercenary and a diamond smuggler.

Maddy, newly arrived in Africa, is researching a big story on the illegal trade in diamonds found in conflict zones. Danny is after the eponymous, emblematic blood diamond, a gem the size of a bird's egg that Solomon has found and buried. Solomon needs Danny's help in recovering his dispersed family. Maddy sees that Danny can provide the evidence that links South Africa, Britain, the States, the Low Countries and India in the illicit diamond business.

Through this trio, the film touches on the chief issues in Africa - post-colonial instability, tribal warfare, commercial exploitation, the brutal conscription and indoctrination of boy soldiers, the indifference of the West to a blighted continent. Danny, as ruthless a killer as the revolutionaries, represents cynicism, shallow realism and the instinct and skill to survive. Gentle, idealistic Solomon stands for hope, peaceful resolution, the warmth of family life; his ambition is to see his son become a doctor. Maddy is there to be instructed, to have her liberal pieties tested and to be one of the agencies through which Danny is redeemed.

She's the audience for a lecture from Danny on the hopeless state of the continent and is introduced to the expression TIA (as the newcomer to the platoon in Saving Private Ryan is taught the acronym Fubar). It means 'This Is Africa', said with a resigned shrug, excusing everything, explaining nothing.

But the film has a romantic aspect and Zwick and his screenwriter Charles Leavitt are eager to show that Danny is as bound to Africa as Solomon is. In addition to the blood-tainted diamond, there is another symbol in the identification of blood with the continent's red soil. Danny's mentor, a mercenary colonel, uses this image of rootedness in a chat they have in a South African vineyard and the pay-off comes when blood and soil are mingled in a final scene that recalls the end of Hemingway's For Whom the Bell Tolls

As in Zwick's previous films, notably Glory, his American Civil War picture about the first black combat unit raised to fight with the Union army, the Gulf war film Courage Under Fire and The Last Samurai, discussion of moral, cultural and racial issues is combined with realistic, large-scale battle scenes. Here, the issues may be simply put, but the brilliant action sequences engage us viscerally, from the opening raid on Solomon's village through the murderous invasion of Freetown to the climactic assault by a mercenary helicopter gunship on a rebel stronghold. In addition to the scenes in Sierra Leone, there are documentary-style recreations of international conferences on the diamond trade and of the process by which the gems are laundered.

The film is superbly photographed by Eduardo Serra, the prolific Franco-Portuguese cinematographer, and there are two interesting pieces of minor casting. The first is Michael Sheen in the brief role of the shifty, unprincipled London representative of an Anglo-South African diamond company, After playing one of the titular Bright Young Things in the Stephen Fry film of Waugh's Vile Bodies, his two performances as Tony Blair and his current stage appearance as David Frost, Sheen is becoming typecast as a dislikable kind of shifty, shallow Englishman.

The other performance is more heartening. Winston Ntshona, one of Africa's greatest actors, is on screen for an indelible couple of minutes as a village elder, the only survivor of a massacre. Who could forget his appearances on stage (and TV) with John Kani in Sizwe Banzi Is Dead and The Island back in the 1970s or their triumph in Waiting for Godot?